The present invention relates to an energy efficient method for production of aluminum metal by reduction with carbon of the oxygen-bound aluminum in Al.sub.2 O.sub.3.
Numerous individuals have previously recognized that considerable energy and capital savings are theoretically obtainable in producing aluminum by direct reduction of Al.sub.2 O.sub.3. Several prior art patents describe processes for production of aluminum by carbothermic reduction. In addition, a few companies have invested heavily in pilot processes aimed at making aluminum by carbothermic reduction. However, there does not yet exist a commercial process for production of aluminum metal by carbothermic reduction in spite of the considerable efforts of prior art investigators.
One major difficulty associated with prior art carbothermic reduction processes is that they have relied upon electrically heated furnaces as a major source of energy. It is well known that when electricity is produced by combustion of fossil fuels, only a minor portion of the heat value of the fuel is recovered as electricity. Accordingly, prior art electrically powered processes for carbothermic reduction of aluminum from Al.sub.2 O.sub.3 are relatively wasteful of energy.
In Cochran U.S. Pat. No. 3,971,653, issued July 27, 1976, one of the inventors of the present invention described a two-stage process for carbothermic production of aluminum by direct reduction of alumina. The present method differs from the process of the Cochran patent in that there is herein described a method using a stack-type reactor for increasing the energy efficiency of the Cochran process and for reducing product losses. The present invention relies upon the same principal chemical reactions underlying the Cochran process, and, to the extent not inconsistent herewith, the disclosure of said U.S. Pat. No. 3,971,653 is incorporated by reference.
Dewing et al U.S. Pat. No. 4,099,959 discloses a carbothermic reduction process for aluminum production having a first step in which aluminum carbide is produced, and a second step in which aluminum carbide and alumina are reacted at a temperature higher than that of the first step to yield aluminum. Heated gaseous carbon monoxide evolved in both steps is used to preheat the reactants. However, rather than providing a stack reactor, Dewing et al prefer to perform the first step in a low temperature zone and the second step in a high temperature zone, with the two zones being at different locations on generally the same level. The reaction is performed by circulating a stream of molten slag through successive low and high temperature zones. The reactants are not heated by partial combustion of carbon so that a major proportion of energy requirements must be met by electricity.
In Kibby U.S. Pat. No. 4,033,757, there is described a carbothermic reduction process carried out in a carbon arc furnace achieving a temperature of about 2100.degree. C., as shown in FIG. 1 of the Kibby patent. The furnace is fed with Al.sub.4 O.sub.4 C (equivalent to 4 Al.sub.2 O.sub.3 +Al.sub.4 C.sub.3) and C in a composite charge having a mole ratio of oxygen to carbon of about 1:1. The arc furnace forms aluminum containing no more than about 10% by weight of aluminum carbide. Volatile products pass upwardly through the composite charge, further reacting to form liquid aluminum or compounds capable of forming liquid aluminum through still other reactions.
In columns 3 and 4, Kibby teaches a prereduction step for converting alumina and coke or coal to Al.sub.4 O.sub.4 C at a temperature of about 1900.degree. to 1950.degree. C. The reactants are shaped into briquettes and placed in a shaft furnace which may be heated electrically or by combustion of additional carbon with oxygen. There is no suggestion in Kibby that the shaft furnace of the prereduction step and the carbon arc furnace of FIG. 1 should be combined to form a single shaft furnace having upper and lower reaction zones for carrying out both steps of the process. The two steps are carried out separately rather than being combined into a single shaft furnace as in the energy efficient method of the present invention.
In Grunert et al U.S. Pat. No. 2,974,032 there is shown a process for carbothermic reduction of alumina wherein an initial stage of the process is carried out in an electric arc furnace at a temperature above 2300.degree. C. An aluminum-aluminum carbide system is tapped from the first stage, and the system is added to a flux at a temperature below 1000.degree. C. for recovery of aluminum. Operation at temperatures above 2300.degree. C. makes the Grunert process less energy efficient than the method of the present invention.
Shiba et al U.S. Pat. No. 2,723,093 discloses a process for recovery of liquid aluminum from an Al-Al.sub.4 C.sub.3 liquid. An electric arc maintains the liquid at a temperature of about 2100.degree. to 2500.degree. C. in a high temperature zone, but recovery of aluminum from the liquid requires formation of a low temperature zone at about 1400.degree. to 1900.degree. C. FIG. 1 of the Shiba et al patent shows a charge of Al.sub.2 O.sub.3 and C at a higher level in an electric furnace than the Al-Al.sub.4 C.sub.3 liquid. However, Shiba et al do not suggest a two-stage process for conversion of the solid charge to the liquid with the first stage being carried out at a lower temperature than the second stage.
It is a principal object of the present invention to provide a method for carbothermic production of aluminum having improved energy efficiency compared with prior art processes.
It is a related object of the present invention to provide a method for carbothermic production of aluminum from Al.sub.2 O.sub.3 wherein only a minor portion of the process energy is provided by electricity, and a major portion is provided by combustion of C.
A further object of the invention is to meet electrical requirements of the method by combustion of byproduct CO.
Another object of the invention is to provide a stack-type reactor having two zones, one above the other, for performing the method of the invention.
Additional objects and advantages of the invention will become apparent to persons skilled in the art from the following specification.